Current source

ABSTRACT

A first resistor is connected between the collector and base of a first transistor of a matched pair and a second resistor is connected in the emitter circuit of a second transistor of the matched pair. The impedance of the first resistor is equal to the impedance of the second resistor. The first resistor is provided for increasing the emitter current of the second transistor to compensate for loss in the base current of the second transistor due to the finite and changing beta characteristics of the transistors.

United States Patent Lucas [54] CURRENT SOURCE [72] inventor: Paul G. Lucas, Sudbury, Mass.

[73] Assignee: Gordon Engineering Company, Wakefield,

Mass.

[22] Filed: Sept. 25, 1970 21 Appl. No.: 75,573

[52] US. Cl ..307/296, 307/264, 330/22 [51] Int. Cl. ..H03k 17/00 [58] Field of Search ..307/264, 270, 297; 330/22; 328/ l 73 [56] References Cited UNITED STATES PATENTS 3,320,439 5/1967 Widlar ..307/297 [451 May 2,1972

3,364,434 1/l968 Widlar ...330/22 3,534,279 l0/l970 Limberg ..330/22 Primary Examiner-John S. Heyman Assistant Examiner-B. P. Davis Attorney-Morse, Altman & Oates [57] ABSTRACT A first resistor is connected between the collector and base of 6 Claims, 2 Drawing Figures PATENTEDMAY 2 m2 3,660,694

FIG.I

Fl G 2 INVENTOR PAUL G. LUCAS ATTORNEYS BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to transistor circuits and more particularly to matched pair transistor circuits for current sources.

2. Description of the Prior Art It is well known in the art of electronics that transistor matched pairs are used to advantage in current sources. In one type of conventional current source, the V of the first transistor compensates the V of the second transistor in order to maintain a constant collector current in the second transistor. However such current sources suffered from errors which have been introduced by the finite and changing beta characteristics of the transistor.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a current source which is characterized by a resistor connected between the collector and base of a first transistor of a matched pair, a reference source connected between the emitter of the first transistor and signal ground, and a second resistor connected between the emitter of a second transistor of the matched pair and signal ground. The impedance of the first resistor is equal to the impedance of the second resistor. The combination of transistor matched pair, first and second resistors, and reference source is such as to increase the emitter current in the second transistor in order to compensate for loss in base current due to the finite value and changing value of the transistor beta, whereby the collector current of the second transistor is maintained at a constant level.

The invention accordingly is comprised of the current source possessing the construction, combination of elements, and arrangement of parts that are exemplified in the following detailed disclosure, the scope of which will be indicated in the appended claims.

BRIEF DESCRIPTION OF DRAWINGS For a fuller understanding of the nature and objects of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of a current source well known in the art; and

FIG. 2 is a schematic diagram of a current source made according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the conventional current source illustrated in FIG. 1, a resistor (R) 10 is connected between a terminal 12 having a positive voltage applied thereto and the collector of a transistor 14. A reference source (E 16 is connected between a signal ground 18 and the emitter of transistor 14. The collector and base of transistor 14, one side of resistor 10, and the base of a transistor are connected together at a common junction 19. A resistor (R 22 is connected between signal ground 18 and the emitter of transistor 20. Transistors l4 and 20 are a matched pair.

In order for the base-emitter voltage 'drop for each of transistors 14 and 20 to be equal and track each other, the impedance of resistor 22 is such that the collector current 1 of transistor 14 is given by the expression:

Since the base-emitter voltage drop of each of the transistors is equal, the voltage across resistor 22 is equal to E and the current 1 through resistor 22 is given by the expression:

I nel 1 Accordingly, the collector current I, of transistor 20 is given by the expression:

where E /BR, is the base current of transistor 20,[l being the base transport function.

The change in base-emitter voltage (AV;,,) is not the subject of this invention and is therefore omitted in all expressions contained herein.

As noted by the foregoing expression for I the collector current through transistor 20 is a function of the transistor B characteristics which has a finite value that changes with temperature. That is, the term E /BR, represents an error factor. In consequence, it will be readily appreciated that the collector current I of transistor 20 is dependent upon the transistor [3 and fluctuates therewith.

Referring now to FIG. 2, the current source embodying the invention is comprised of a matched pair of semi-conductors 24 and 26; impedance 28, like impedance 30 and 32; and a reference source (E 34. By way of example, semi-conductors 24 and 26 are NPN transistors. It is to be understood that, in alternate embodiments semi-conductors 24 and 26 are other than NPN transistors, for example PNP transistors. Of course, when transistors 24 and 26 are PNP, the voltage polarities are opposite to that shown in FIG. 2.

Impedance 28, for example a resistor R is connected between a terminal 36 having a positive voltage applied thereto and the collector of transistor 24. Reference source 34 is connected between the emitter of transistor 24 and a reference signal 38. The base and collector of transistor 24 are connected through impedance 30, for example a resistor R The junction of one side of each resistor 28 and 30, and the collector of transistor 24 is denoted by reference character 40. The base of transistor 26 is connected to junction 40 and the emitter of transistor 26 is connected to signal ground 38 through impedance 32, for example a resistor R The junction of one side of resistor 32 and reference signal 38 is denoted by reference character 44 and the junction of the other side of resistor 32 and the emitter of transistor 26 is denoted by reference character 46. For convenience, in the following description, junctions 40, 44, and 46 are denoted by reference characters A, B, and C, respectively.

In order for the base-emitter voltage drop for each of transistors 24 and 26 to be equal and track each other, the impedance of resistor 22 is such that the collector current I of transistor 24 is given by the expression:

. E /R and the base current 1,, of transistor 24 is given by expression:

0 li/[ 4 The compensating voltage drop V across resistor 30 is given by the expression:

The voltage E between junctions 40 and 44 is given by ex pression:

Accordingly, the current I through resistor 32 is given by expression:

I M=ia & 1 R R [3R The collector current I of transistor 26 is given by the expression:

ca 1 br where I is the base current of transistor 26.

Since transistors 24 and 26 are a matched pair, I, 1 and the expression for 1 may be written:

1 I 1,, Substituting the expressions for I and 1,, in the expression for 1, yields:

As noted by the foregoing expression for I the collector current of transistor 26 is independent of the transistor ,8 characteristic. In other words, the term E /BR which corresponds to the term E /fill? in the expression for the collectorcurrent of the conventional current source of FIG. 1, has been canceled by the addition of resistor 30 between the base and collector of transistor 24, resistor 30 being the same value as resistor 32 in the emitter circuit of transistor 26. That is, in the illustrated example, voltage E is increased by an amount equal to E lfi, in consequence the emitter current of transistor 26 is increased to compensate for the loss in base current due to the finite and changing transistor B. v

Since certain changes may be. made in the foregoing disclosure without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description and depicted in the accompanying drawings be construed in an illustrative and not in a limiting sense.

What is claimed is:

1. A current source comprising:

a. first and second semi-conductor means, said first and second semi-conductor means being a matched pair, each of said semi-conductor means having a plurality of terminal means;

b. first impedance means;

c. first means connected to one side of said first impedance means, the other side of said first impedance means connected to first terminal means of said first semi-conductor means, said first means adapted for reception of a voltage;

d. second impedance means connected between said first terminal means of said first semi-conductor means and second terminal means of said first semi-conductor means; i

e. reference sourse means connected between a'third terminal means of said first semi-conductor means and a reference signal; and

f. third impedance means connected between said reference signal and third terminal means of said second semi-conductor means, said third impedance means substantially equal to said second impedance means, second terminal means of said second semi-conductor means connected to said first terminal means of said first semi-conductor means, a constant current flowing in first terminal means of said second semi-conductor means. 2. The current source as claimed in claim 1 wherein each of said first and second semi-conductor means is NPN transistor means.

3. The current source as claimed in claim 2 wherein said first, second, and third terminal means of said first transistor means is the collector, base, and emitter, respectively, and said first, second, and third terminal means of said second transistor means is the collector, base, and emitter, respectively.

4. A current source comprising:

a. first and second transistor means, said first and second transistor means being a matched pair, said first and second transistors provided with base, collector and emitter terminals;

b. first means electrically connected to said first and second transistor means, said first means adapted for reception of a voltage;

c. reference source means connected between the emitter of said first transistor means and a reference signal;

first and second impedance means, said first and second impedance means having substantially equal ohmic values, said first impedance means connected between the base and collector of said first transistor means, said second impedance means connected between the emitter of said second transistor means and said reference signal, the collector current of said second transistor means being constant.

5. The current source as claimed in claim 4 wherein each of said firstand second transistor means is NPN transistor means. 6. An improved current source of the type wherein the base and collector of a first transistor are connected together, the collector of the first transistor is connected to the base of a second transistor, the first and second transistor being a matched pair, a voltage is applied to the collector of the first transistor through a first resistor, a reference source is connected between a signal ground and the emitter of the first transistor, a second resistor is connected between the emitter of the second transistor and the signal ground, and wherein the improvement comprises a third resistor serially connected between the base and collector of the first transistor, said second and third resistors being substantially equal.

* 1F t l 

1. A current source comprising: a. first and second semi-conductor means, said first and second semi-conductor means being a matched pair, each of said semiconductor means having a plurality of terminal means; b. first impedance means; c. first means connected to one side of said first impedance means, the other side of said first impedance means connected to first terminal means of said first semi-conductor means, said first means adapted for reception of a voltage; d. second impedance means connected between said first terminal means of said first semi-conductor means and second terminal means of said first semi-conductor means; e. reference sourse means connected between a third terminal means of said first semi-conductor means and a reference signal; and f. third impedance means connected between said reference signal and third terminal means of said second semi-conductor means, said third impedance means substantially equal to said second impedance means, second terminal means of said second semiconductor means connected to said first terminal means of said first semi-conductor means, a constant current flowing in first terminal means of said second semi-conductor means.
 2. The current source as claimed in claim 1 wherein each of said first and second semi-conductor means is NPN transistor means.
 3. The current source as claimed in claim 2 wherein said first, second, and third terminal means of said first transistor means is the collector, base, and emitter, respectively, and said first, second, and third terminal means of said second transistor means is the collector, base, and emitter, respectively.
 4. A current source comprising: a. first and second transistor means, said first and second transistor means being a matched pair, said first and second transistors provided with base, collector and emitter terminals; b. first means electrically connected to said first and second transistor means, said first means adapted for reception of a voltage; c. reference source means connected between the emitter of said first transistor means and a reference signal; d. first and second impedance means, said first and second impedance means having substantially equal ohmic values, said first impedance means connected between the base and collector of said first transistor means, said second impedance means connected between the emitter of said second transistor means and said reference signal, the collector current of said second transistor means being constant.
 5. The current source as claimed iN claim 4 wherein each of said first and second transistor means is NPN transistor means.
 6. An improved current source of the type wherein the base and collector of a first transistor are connected together, the collector of the first transistor is connected to the base of a second transistor, the first and second transistor being a matched pair, a voltage is applied to the collector of the first transistor through a first resistor, a reference source is connected between a signal ground and the emitter of the first transistor, a second resistor is connected between the emitter of the second transistor and the signal ground, and wherein the improvement comprises a third resistor serially connected between the base and collector of the first transistor, said second and third resistors being substantially equal. 